Historical city reconstruction workflow
You could try to export from OpenStreetMap
How to model effectively using exact measurements?
It depends on what you want to achieve (should be another question) you could use layers and groups.
For buildings in the background adding textures should be sufficient for close up I would refine the geometry and add more materials. Related: Trying to create a script that makes roofs on selected boxes


Imported .osm after extrusion (Note that exporting a whole city leads to 2GB files which can't be loaded in Blender)



For some cities 360° images are available from Google street view / maps view which help to elaborate on building details and estimate their age.



Check this awesome tutorial Blender tutorial: How to Create a City